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- 1H-NMR Relaxometry (1)
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- hydrophobicity (1)
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- repellency (1)
- soil organic matter (1)
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Institute
- Institut für Umweltwissenschaften (3) (remove)
Organic substances play an essential role for the formation of stable soil structures. In this context, their physico-chemical properties, interactions with mineral soil constituents and soil-water interactions are particu-larly important. However, the underlying mechanisms contributing to soil particle cementation by swollen or-ganic substances (hydrogels) remains unclear. Up to now, no mechanistic model is available which explains the mechanisms of interparticulate hydrogel swelling and its contribution to soil-water interactions and soil structur-al stability. This mainly results from the lack of appropriate testing methods to study hydrogel swelling in soil as well as from the difficulties of adapting available methods to the system soil/hydrogel.
In this thesis, 1H proton nuclear magnetic resonance (NMR) relaxometry was combined with various soil micro- and macrostructural stability testing methods in order to identify the contribution of hydrogel swelling-induced soil-water interactions to the structural stability of water-saturated and unsaturated soils. In the first part, the potentials and limitations of 1H NMR relaxometry to enlighten soil structural stabilization mechanism and vari-ous water populations were investigated. In the second part, 1H-NMR relaxometry was combined with rheologi-cal measurements of soil to assess the contribution of interparticulate hydrogel swelling and various polymer-clay interactions on soil-water interactions and soil structural stability in an isolated manner. Finally, the effects of various organic and mineral soil fractions on soil-water interactions and soil structural stability was assessed in more detail for a natural, agriculturally cultivated soil by soil density fractionation and on the basis of the experiences gained from the previous experiments.
The increased experiment complexity in the course of this thesis enabled to link physico-chemical properties of interparticulate hydrogel structures with soil structural stability on various scales. The established mechanistic model explains the contribution of interparticulate hydrogels to the structural stability of water-saturated and unsaturated soils: While swollen clay particles reduce soil structural stability by acting as lubricant between soil particles, interparticulate hydrogel structures increase soil structural stability by forming a flexible polymeric network which interconnects mineral particles more effectively than soil pore- or capillary water. It was appar-ent that soil structural stability increases with increasing viscosity of the interparticluate hydrogel in dependence on incubation time, soil texture, soil solution composition and external factors in terms of moisture dynamics and agricultural management practices. The stabilizing effect of interparticulate hydrogel structures further in-crease in the presence of clay particles which is attributed to additional polymer-clay interactions and the incor-poration of clay particles into the three-dimensional interparticulate hydrogel network. Furthermore, the simul-taneous swelling of clay particles and hydrogel structures results in the competition for water and thus in a mu-tual restriction of their swelling in the interparticle space. Thus, polymer-clay interactions not only increase the viscosity of the interparticulate hydrogel and thus its ability to stabilize soil structures but further reduce the swelling of clay particles and consequently their negative effects on soil structural stability. The knowledge on these underlying mechanisms enhance the knowledge on the formation of stable soil structures and enable to take appropriate management practices in order to maintain a sustainable soil structure. The additionally out-lined limitations and challenges of the mechanistic model should provide information on areas with optimization and research potential, respectively.
Abstract The present work investigates the wetting characteristics of soils with regard to their dependence on environmental parameters such as water content (WC), pH, drying temperature and wetting temperature of wettable and repellent soils from two contrasting anthropogenic sites, the former sewage disposal field Berlin-Buch and the inner-city park Berlin-Tiergarten. The aim of this thesis is to deepen the understanding of processes and mechanisms leading to changes in soil water repellency. This helps to gain further insight into the behaviour of soil organic matter (SOM) and identifying ways to prevent or reduce the negative effects of soil water repellency (SWR). The first focus of this work is to determine whether chemical reactions are required for wetting repellent samples. This hypothesis was tested by time and temperature dependence of sessile drop spreading on wettable and repellent samples. Additionally, diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy was used to determine whether various drying regimes cause changes in the relative abundance of hydrophobic and hydrophilic functional groups in the outer layer of soil particles and whether these changes can be correlated with water content and the degree of SWR. Finally, by artificially altering the pH in dried samples applying acidic and alkaline reagents in a gaseous state, the influence of only pH on the degree of SWR was investigated separately from the influence of changes in moisture status. The investigation of the two locations Buch and Tiergarten, each exceptionally different in the nature of their respective wetting properties, leads to new insights in the variety of appearance of SWR. The results of temperature, water content and pH dependency of SWR on the two contrasting sites resulted in one respective hypothetical model of nature of repellency for each site which provides an explanation for most of the observations made in this and earlier studies: At the Tiergarten site, wetting characteristics are most likely determined by micelle-like arrangement of amphiphiles which depends on the concentration of water soluble amphiphilic substances, pH and ionic strength in soil solution. At low pH and at high ionic strength, repulsion forces between hydrophilic charged groups are minimized allowing their aggregation with outward orientated hydrophobic molecule moieties. At high pH and low ionic strength, higher repulsion forces between hydrophilic functional groups lead to an aggregation of hydrophobic groups during drying, which results in a layer with outward oriented hydrophilic moieties on soil organic matter surface leading to enhanced wettability. For samples from the Buch site, chemical reactions are necessary for the wetting process. The strong dependence of SWR on water content indicates that hydrolysis-condensation reactions are the controlling mechanisms. Since acid catalyzed hydrolysis is an equilibrium reaction dependent on water content, an excess of water favours hydrolysis leading to an increasing number of hydrophilic functional groups. In contrast, water deficiency favours condensation reactions leading to a reduction of hydrophilic functional groups and thus a reduction of wettability. The results of the present investigation and its comparison with earlier investigations clearly show that SWR is subject to numerous antagonistically and synergistically interacting environmental factors. The degree of influence, which a single factor exerts on SWR, is site-specific, e.g., it is dependent on special characteristics of mineral constituents and SOM which underlies the influence of climate, soil texture, topography, vegetation and the former and current use of the respective site.
Soil organic matter (SOM) is a key component responsible for sequestration of organic molecules in soil and regulation of their mobility in the environment. The basic structure of SOM is a supramolecular assembly responding dynamically to the environmental factors and the presence of interacting molecules. Despite of the advances in the understanding of sorption processes, the relation between sorbate molecules, SOM supramolecular structure and its dynamics is limited. An example of a dynamic nature of SOM is a physicochemical matrix aging that is responsible for SOM structural arrangement. The underlying process of the physicochemical aging is the formation of water molecule bridges (WaMB) between functional groups of molecular segments. Since WaMB influence the stiffness of SOM structure, it was hypothesized that formation of WaMB contributes to the sequestration of organic molecules. However, this hypothesis has not been tested experimentally until now. Furthermore, the knowledge about the influence of organic molecules on WAMB is based solely on computer modeling studies. In addition, the influence of organic molecules on some physical phases forming SOM is not well understood. Especially, the interactions between organic molecules and crystalline phases represented by aliphatic crystallites, are only presumed. Thus, the investigation of those interactions in unfractioned SOM is of high importance.
In order to evaluate the involvement of WaMB in the sequestration of organic molecules and to increase our understanding about interactions of organic chemicals with WaMB or aliphatic crystallites, the following hypotheses were tested experimentally. 1) Similarly to crystalline phases in synthetic polymers, aliphatic crystallites, as a part of SOM, cannot be penetrated by organic molecules. 2) The stability of WaMB is determined by the ability of surrounding molecules to interact with water forming WaMB. 3) WaMB prevent organic molecules to leave the SOM matrix and contribute thus to their immobilization. In order to test the hypotheses 1 and 2, a set of experiments including treatment of soils with chosen chemicals was prepared. Interaction abilities of these chemicals were characterized using interaction parameters from the Linear Solvation Energy Relationship theory. WaMB characteristics were monitored using Differential Scanning Calorimetry (DSC) allowing to measure the WaMB thermal stability and the rigidity of SOM matrix; which in turn was determined by the heat capacity change. In addition, DSC and 13C NMR spectroscopy assessed thermal properties and the structure of aliphatic crystallites. The spiking of samples with a model compound, phenol, and measurements of its desorption allowed to link parameters of the desorption kinetics with WaMB characteristics.
The investigation showed that the WaMB stability is significantly reduced by the presence of molecules with H-donor/acceptor interaction abilities. The matrix rigidity associated with WaMB was mainly influenced by the McGowan’s volume of surrounding molecules, suggesting the importance of dispersion forces. The desorption kinetics of phenol followed a first order model with two time constants. Both of them showed a relation with WaMB stability, which supports the hypothesis that WaMB contribute to the physical immobilization of organic molecules. The experiments targeted to the crystallites revealed their structural change from the ordered to the disordered state, when in contact with organic chemicals. This manifested in their melting point depression and the decrease of overall crystallinity. Described structural changes were caused by molecules interacting with specific as well as non-specific forces, which suggests that aliphatic crystallites can be penetrated and modified by molecules with a broad range of interaction abilities.
This work shows that chosen organic molecules interact with constituents of SOM as exemplified on WaMB and aliphatic crystallites, and cause measurable changes of their structure and properties. These findings show that the relevance of aliphatic crystallites for sorption in soil may have been underestimated. The results support the hypothesis that physicochemical matrix aging significantly contributes to the immobilization of organic chemicals in SOM.